In recent years, a technique on an optical coherence tomography (OCT) is widely used in a field of funduscopic examination or the like. The OCT is a technique capable of non-invasively imaging a tomographic image of a living body skin of 1 to 2 mm-depth with a spatial resolution of about 10 μm. The OCT is based on an interferometer using low coherent light. Further, the OCT selectively detects a straight advancing light component obtained by radiation of light on a living tissue and reflection of the light from an inside of the tissue, and forms a two-dimensional or three-dimensional tomographic image, based on the detection.
FIG. 8 is a conceptual diagram illustrating a basic configuration of the OCT (e.g. see NPL 1). As illustrated in FIG. 8, the basic configuration of the OCT includes a light source 910, a reference light mirror 920, a beam splitter 930, and a light detector 940. A measured object MO is a living body, for example.
The light source 910 emits low coherent light in a near-infrared range. Low coherent light is light whose timewise coherence is extremely low. Note that the light source 910 includes a super luminescent diode (SLD), for example.
As illustrated in FIG. 8, the light source 910 emits low coherent light toward the beam splitter 930. Low coherent light in this case has a central wavelength λC=850 nm, and a spectral full width at half maximum Δλ=20 nm, for example.
Light source light is split into two beams by the beam splitter 930. One beam of the split light source light is directed toward the reference mirror 920, is reflected on the reference mirror 920, and then returns to the beam splitter 930 as reference light ER. The other beam of the split light source light is radiated on the measured object MO as measurement light. A multitude of beams of reflection light (signal light) ES (reflection light ESA and ESB, as an example) from a surface and an inside of the measured object MO return to the beam splitter 930. A half of reference light ER and a half of reflection light ES that return to the beam splitter 930 have optical paths coincident with each other, and interfere with each other on the front side of the light detector 940.
Herein, as illustrated in FIG. 8, specific reflection surfaces on the surface and the inside of the measured object MO along a propagation direction of signal light are defined as A and B.
When it is assumed that the reflection surface A inside the measured object MO, and a position 1 of the reference light mirror 920 are optically equidistant with respect to the beam splitter 930, time zones of sinusoidal vibration of reference light ER1 and reflection light ESA are superimposed each other, and the reference light ER1 and the reflection light ESA interfere with each other. Consequently, the detector 940 obtains interference light between the reference light ER1 and the reflection light ESA. Next, in order to obtain interference light from reflection light ESB, the reference light mirror 920 is moved in a direction to be away from the beam splitter 930 up to a position 2 where a reflection point B and the reference light mirror 920 are equidistant. In this way, sequentially moving the reference light mirror 920 to allow the detector 940 to detect interference light makes it possible to obtain a reflection light intensity distribution in an optical axis direction. As described above, the OCT is generally used for tomographic imaging. Although light interference is used, the OCT is not positively used for deriving a refractive index of a measured object.
Note that PTLs 1 to 4 also disclose a technique associated with the present invention.